1. Field of the Invention
The present invention relates to an image sensor, and more specifically to a camera-type sensor capable of providing a video signal, that is, the equivalent of an image sequence with a 25- or 30-Hz frequency corresponding to standard video signal frequencies.
2. Discussion of the Related Art
An example of an image sensor is described in U.S. Pat. No. 6,271,785. As shown in FIG. 1 of this patent, the image sensor comprises an array of pixels arranged in rows and columns. Each pixel comprises a photodetector connected to an input of a comparator and a switch placed between the comparator output and a column line, the switch being controlled by a row line (ROW1, ROW2 . . . ). The output of a digital-to-analog converter (D/A) is connected to the second input of the comparator of each pixel. The D/A converter is controlled by a counter. Each column line is connected to a register.
Conventionally, the rows are read one after the other. On reading a row, a voltage ramp is generated by the D/A converter. For each pixel in the considered row, the output of its comparator switches when the ramp voltage becomes equal to the voltage provided by its photodetector. When the comparator of a pixel of a given column switches, the register connected to this column stores the digital value transmitted by the counter.
A disadvantage of the previously-described image sensor is that the time necessary to read each line is relatively long. The number of sensor lines must thus be limited in the case where images are desired to be provided with a video frequency.
Further, this sensor is not provided to take snapshots. Referring to FIG. 2, this would require simultaneously performing for each pixel a charge transfer from the photodiode to the comparator input, through transistor M2. The pixels would then be read row by row according to the above-mentioned method. Since the voltage “stored” on the comparator input tends to “deteriorate” relatively fast, the values read for the last read pixels would be tainted with error, given the long read time necessary to scan all of the pixels.